Synchronization of asymmetric active-active, asynchronously-protected storage

ABSTRACT

In one aspect, a method includes synchronizing a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity. In another aspect, an apparatus includes electronic hardware circuitry configured to synchronize a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity. In a further aspect, an article includes a non-transitory computer-readable medium that stores computer-executable instructions. The instructions cause a machine to synchronize a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity.

BACKGROUND

Computer data is vital to today's organizations and a significant part of protection against disasters is focused on data protection. As solid-state memory has advanced to the point where cost of memory has become a relatively insignificant factor, organizations can afford to operate with systems that store and process terabytes of data.

Conventional data protection systems include tape backup drives, for storing organizational production site data on a periodic basis. Another conventional data protection system uses data replication, by creating a copy of production site data of an organization on a secondary backup storage system, and updating the backup with changes. The backup storage system may be situated in the same physical location as the production storage system, or in a physically remote location. Data replication systems generally operate either at the application level, at the file system level, or at the data block level.

SUMMARY

In one aspect, a method includes synchronizing a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity. In another aspect, an apparatus includes electronic hardware circuitry configured to synchronize a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity. In a further aspect, an article includes a non-transitory computer-readable medium that stores computer-executable instructions. The instructions cause a machine to synchronize a second storage volume to a first storage volume by synchronizing extents according to priority based on extent activity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a data protection system.

FIG. 2 is an illustration of an example of a journal history of write transactions for a storage system.

FIG. 3 is a block diagram of an example of a replication system that includes storage virtualization and provides asymmetric active-active access of asynchronously-protected data storage.

FIGS. 4A and 4B are flowcharts of an example of a process to account for a lag in a replication system having an asymmetric active-active access of asynchronously-protected data storage.

FIG. 4C is a flowchart of an example of a process to determine whether a replicated storage may be read.

FIG. 5 is a flowchart of an example of a process to transfer primary storage functionality from one site to another site.

FIG. 6 is a flowchart of an example of a process to automatically (without user intervention) transfer primary storage functionality from one site to another site.

FIG. 7 is a diagram to illustrate synchronization of extents between a primary storage volume and a replication storage volume.

FIG. 8 is a flowchart of an example of a process to handle extents during synchronization.

FIG. 9 is a flowchart of an example of a process to select an extent for synchronization.

FIG. 10 is a flowchart of an example of a process to read from the second (replication) site.

FIG. 11 is a computer on which any of the processes of FIGS. 4A to 4C, 5, 6 and 8 to 11 may be implemented.

DETAILED DESCRIPTION

Described herein is an approach to synchronize an asymmetric Active-Active (A/A) data storage by delaying synchronization of areas in the storage array that are write active (i.e., having writes to those areas of storage area) and giving priority for synchronization to those areas that are not write active which are less likely to be overwritten during the time of synchronization of the entire storage array. For example, a synchronization process that ignores usage of each of the areas of the storage area can lead to needlessly sending over a considerable amount of data. Since the storage area is not considered consistent until the synchronization process is complete, continuously updating active regions that change is inefficient. The approach described herein reduces the amount of data that needs to be sent over distance and reduces synchronization time, thus reducing a recovery point objective (RPO) and allows efficient local reads from the replica site, while the synchronization process is active.

The following definitions may be useful in understanding the specification and claims.

BACKUP SITE—a facility where replicated production site data is stored; the backup site may be located in a remote site or at the same location as the production site;

BOOKMARK—a bookmark is metadata information stored in a replication journal which indicates a point in time.

DATA PROTECTION APPLIANCE (DPA)—a computer or a cluster of computers responsible for data protection services including inter alia data replication of a storage system, and journaling of I/O requests issued by a host computer to the storage system;

HOST—at least one computer or networks of computers that runs at least one data processing application that issues I/O requests to one or more storage systems; a host is an initiator with a SAN;

HOST DEVICE—an internal interface in a host, to a logical storage unit;

IMAGE—a copy of a logical storage unit at a specific point in time;

INITIATOR—a node in a SAN that issues I/O requests;

I/O REQUEST—an input/output request (sometimes referred to as an I/O), which may be a read I/O request (sometimes referred to as a read request or a read) or a write I/O request (sometimes referred to as a write request or a write);

JOURNAL—a record of write transactions issued to a storage system; used to maintain a duplicate storage system, and to roll back the duplicate storage system to a previous point in time;

LOGICAL UNIT—a logical entity provided by a storage system for accessing data from the storage system. The logical disk may be a physical logical unit or a virtual logical unit;

LUN—a logical trait number for identifying a logical unit;

PHYSICAL LOGICAL UNIT—a physical entity, such as a disk or an array of disks, for storing data in storage locations that can be accessed by address;

PRODUCTION SITE—a facility where one or more host computers run data processing applications that write data to a storage system and read data from the storage system;

REMOTE ACKNOWLEDGEMENTS—an acknowledgement from remote DPA to the local DPA that data arrived at the remote DPA (either to the appliance or the journal)

SPLITTER ACKNOWLEDGEMENT—an acknowledgement from a DPA to the protection agent (splitter) that data has been received at the DPA; this may be achieved by an SCSI status command.

SAN—a storage area network of nodes that send and receive an I/O and other requests, each node in the network being an initiator or a target, or both an initiator and a target;

SOURCE SIDE—a transmitter of data within a data replication workflow, during normal operation a production site is the source side; and during data recovery a backup site is the source side, sometimes called a primary side;

STORAGE SYSTEM—a SAN entity that provides multiple logical units for access by multiple SAN initiators

TARGET—a node in a SAN that replies to I/O requests;

TARGET SIDE—a receiver of data within a data replication workflow; during normal operation a back site is the target side, and during data recovery a production site is the target side, sometimes called a secondary side;

VIRTUAL LOGICAL UNIT—a virtual storage entity which is treated as a logical unit by virtual machines;

WAN—a wide area network that connects local networks and enables them to communicate with one another, such as the Internet.

A description of journaling and some techniques associated with journaling may be described in the patent titled “METHODS AND APPARATUS FOR OPTIMAL JOURNALING FOR CONTINUOUS DATA REPLICATION” and with U.S. Pat. No. 7,516,287, which is hereby incorporated by reference.

Before describing a distributed global cache coherence engine in an asynchronous directional replication system, an example replication system is first described in FIGS. 1 and 2.

An Example of a Replication System

Referring to FIG. 1, a data protection system 100 includes two sites; Site I, which is a production site, and Site II, which is a backup site or replica site. Under normal operation the production site is the source side of system 100, and the backup site is the target side of the system. The backup site is responsible for replicating production site data. Additionally, the backup site enables roll back of Site I data to an earlier pointing time, which may be used in the event of data corruption of a disaster, or alternatively in order to view or to access data from an earlier point in time.

FIG. 1 is an overview of a system for data replication of either physical or virtual logical units. Thus, one of ordinary skill in the art would appreciate that in a virtual environment a hypervisor, in one example, would consume logical units and generate a distributed file system on them such as VMFS creates files in the file system and expose the files as logical units to the virtual machines (each VMDK is seen as a SCSI device by virtual hosts). In another example, the hypervisor consumes a network based file system and exposes files in the NFS as SCSI devices to virtual hosts.

During normal operations, the direction of replicate data flow goes from source side to target side. It is possible, however, for a user to reverse the direction of replicate data flow, in which case Site I starts to behave as a target backup site, and Site H starts to behave as a source production site. Such change of replication direction is referred to as a “failover”. A failover may be performed in the event of a disaster at the production site, or for other reasons. In some data architectures, Site I or Site II behaves as a production site for a portion of stored data, and behaves simultaneously as a backup site for another portion of stored data. In some data architectures, a portion of stored data is replicated to a backup site, and another portion is not.

The production site and the backup site may be remote from one another, or they may both be situated at a common site, local to one another. Local data protection has the advantage of minimizing data lag between target and source, and remote data protection has the advantage is being robust in the event that a disaster occurs at the source side,

The source and target sides communicate via a wide area network (WAN) 128, although other types of networks may be used.

Each side of system 100 includes three major components coupled via a storage area network (SAN); namely, (i) a storage system, (ii) a host computer, and (iii) a data protection appliance (DPA). Specifically with reference to FIG. 1, the source side SAN includes a source host computer 104, a source storage system 108, and a source DPA 112. Similarly, the target side SAN includes a target host computer 116, a target storage system 120, and a target DPA 124. As well, the protection agent (sometimes referred to as a splitter) may run on the host, or on the storage, or in the network or at a hypervisor level, and that DPAs are optional and DPA code may run on the storage array too, or the DPA 124 may run as a virtual machine.

Generally, a SAN includes one or more devices, referred to as “nodes”. A node in a SAN may be an “initiator” or a “target”, or both. An initiator node is a device that is able to initiate requests to one or more other devices; and a target node is a device that is able to reply to requests, such as SCSI commands, sent by an initiator node. A SAN may also include network switches, such as fiber channel switches. The communication links between each host computer and its corresponding storage system may be any appropriate medium suitable for data transfer, such as fiber communication channel links.

The host communicates with its corresponding storage system using small computer system interface (SCSI) commands.

System 100 includes source storage system 108 and target storage system 120. Each storage system includes physical storage units for storing data, such as disks or arrays of disks. Typically, storage systems 108 and 120 are target nodes. In order to enable initiators to send requests to storage system 108, storage system 108 exposes one or more logical units (LU) to which commands are issued. Thus, storage systems 108 and 120 are SAN entities that provide multiple logical units for access by multiple SAN initiators.

Logical units are a logical entity provided by a storage system, for accessing data stored in the storage system. The logical unit may be a physical logical unit or a virtual logical unit. A logical unit is identified by a unique logical unit number (LUN). Storage system 108 exposes a logical unit 136, designated as LU A, and storage system 120 exposes a logical unit 156, designated as LU B.

LU B is used for replicating LU A. As such, LU B is generated as a copy of LU A. In one embodiment, LU B is configured so that its size is identical to the size of LU A. Thus for LU A, storage system 120 serves as a backup for source side storage system 108. Alternatively, as mentioned hereinabove, some logical units of storage system 120 may be used to back up logical units of storage system 108, and other logical units of storage system 120 may be used for other purposes. Moreover, there is symmetric replication whereby some logical units of storage system 108 are used for replicating logical units of storage system 120, and other logical units of storage system 120 are used for replicating other logical units of storage system 108.

System 100 includes a source side host computer 104 and a target side host computer 116. A host computer may be one computer, or a plurality of computers, or a network of distributed computers, each computer may include inter alia a conventional CPU, volatile and non-volatile memory, a data bus, an I/O interface, a display interface and a network interface. Generally a host computer runs at least one data processing application, such as a database application and an e-mail server.

Generally, an operating system of a host computer creates a host device for each logical unit exposed by a storage system in the host computer SAN. A host device is a logical entity in a host computer, through which a host computer may access a logical unit. Host device 104 identifies LU A and generates a corresponding host device 140, designated as Device A, through which it can access LU A. Similarly, host computer 116 identifies LU B and generates a corresponding device 160, designated as Device B.

In the course of continuous operation, host computer 104 is a SAN initiator that issues I/O requests (write/read operations) through host device 140 to LU A using, for example, SCSI commands. Such requests are generally transmitted to LU A with an address that includes a specific device identifier, an offset within the device, and a data size. Offsets are generally aligned to 512 byte blocks. The average size of a write operation issued by host computer 104 may be, for example, 10 kilobytes (KB); i.e., 20 blocks. For an I/O rate of 50 megabytes (MB) per second, this corresponds to approximately 5,000 write transactions per second.

System 100 includes two data protection appliances, a source side DPA 112 and a target side DPA 124. A DPA performs various data protection services, such as data replication of a storage system, and journaling of I/O requests issued by a host computer to source side storage system data. As explained in detail herein, when acting as a target side DPA, a DPA may also enable roll back of data to an earlier point in time, and processing of rolled back data at the target site. Each DPA 112 and 124 is a computer that includes inter alia one or more conventional CPUs and internal memory.

For additional safety precaution, each DPA is a cluster of such computers. Use of a cluster ensures that if a DPA computer is down, then the DPA functionality switches over to another computer. The DPA computers within a DPA cluster communicate with one another using at least one communication link suitable for data transfer via fiber channel or IP based protocols, or such other transfer protocol. One computer from the DPA cluster serves as the DPA leader. The DPA cluster leader coordinates between the computers in the cluster, and may also perform other tasks that require coordination between the computers, such as load balancing.

In the architecture illustrated in FIG. 1, DPA 112 and DPA 124 are standalone devices integrated within a SAN. Alternatively, each of DPA 112 and DPA 124 may be integrated into storage system 108 and storage system 120, respectively, or integrated into host computer 104 and host computer 116, respectively. Both DPAs communicate with their respective host computers through communication lines such as fiber channels using, for example, SCSI commands or any other protocol.

DPAs 112 and 124 are configured to act as initiators in the SAN; i.e., they can issue I/O requests using, for example, SCSI commands, to access logical units on their respective storage systems. DPA 112 and DPA 124 are also configured with the necessary functionality to act as targets; i.e., to reply to I/O requests, such as SCSI commands, issued by other initiators in the SAN, including inter alia their respective host computers 104 and 116. Being target nodes, DPA 112 and DPA 124 may dynamically expose or remove one or more logical units.

As described hereinabove, Site I and Site II may each behave simultaneously as a production site and a backup site for different logical units. As such, DPA 112 and DPA 124 may each behave as a source DPA for some logical units, and as a target DPA for other logical units, at the same time.

Host computer 104 and host computer 116 include protection agents 144 and 164, respectively. Protection agents 144 and 164 intercept SCSI commands issued by their respective host computers, via host devices to logical units that are accessible to the host computers. A data protection agent may act on an intercepted SCSI commands issued to a logical unit, in one of the following ways: send the SCSI commands to its intended logical unit; redirect the SCSI command to another logical unit; split the SCSI command by sending it first to the respective DPA; after the DPA returns an acknowledgement, send the SCSI command to its intended logical unit; fail a SCSI command by returning an error return code; and delay a SCSI command by not returning an acknowledgement to the respective host computer.

A protection agent may handle different SCSI commands, differently, according to the type of the command. For example, a SCSI command inquiring about the size of a certain logical unit may be sent directly to that logical unit, while a SCSI write command may be split and sent first to a DPA associated with the agent. A protection agent may also change its behavior for handling SCSI commands, for example as a result of an instruction received from the DPA.

Specifically, the behavior of a protection agent for a certain host device generally corresponds to the behavior of its associated DPA with respect to the logical unit of the host device. When a DPA behaves as a source site DPA for a certain logical unit, then during normal course of operation, the associated protection agent splits I/O requests issued by a host computer to the host device corresponding to that logical unit. Similarly, when a DPA behaves as a target device for a certain logical unit, then during normal course of operation, the associated protection agent fails I/O requests issued by host computer to the host device corresponding to that logical unit.

Communication between protection agents and their respective DPAs may use any protocol suitable for data transfer within a SAN, such as fiber channel, or SCSI over fiber channel. The communication may be direct, or via a logical unit exposed by the DPA. Protection agents communicate with their respective DPAs by sending SCSI commands over fiber channel.

Protection agents 144 and 164 are drivers located in their respective host computers 104 and 116. Alternatively, a protection agent may also be located in a fiber channel switch, or in any other device situated in a data path between a host computer and a storage system or on the storage system itself. In a virtualized environment, the protection agent may run at the hypervisor layer or in a virtual machine providing a virtualization layer.

What follows is a detailed description of system behavior under normal production mode, and under recovery mode.

In production mode DPA 112 acts as a source site DPA for LU A. Thus, protection agent 144 is configured to act as a source side protection agent; i.e., as a splitter for host device A. Specifically, protection agent 144 replicates SCSI I/O write requests. A replicated SCSI I/O write request is sent to DPA 112. After receiving an acknowledgement from DPA 124, protection agent 144 then sends the SCSI I/O write request to LU A. After receiving a second acknowledgement from storage system 108 host computer 104 acknowledges that an I/O command complete.

When DPA 112 receives a replicated SCSI write request from data protection agent 144, DPA 112 transmits certain I/O information characterizing the write request, packaged as a “write transaction”, over WAN 128 to DPA 124 on the target side, for journaling and for incorporation within target storage system 120.

DPA 112 may send its write transactions to DPA 124 using a variety of modes of transmission, including inter alia (i) a synchronous mode, (ii) an asynchronous mode, and (iii) a snapshot mode. In synchronous mode, DPA 112 sends each write transaction to DPA 124, receives back an acknowledgement from DPA 124, and in turns sends an acknowledgement back to protection agent 144. Protection agent 144 waits until receipt of such acknowledgement before sending the SCSI write request to LU A.

In asynchronous mode, DPA 112 sends an acknowledgement to protection agent 144 upon receipt of each I/O request, before receiving an acknowledgement back from DPA 124.

In snapshot mode, DPA 112 receives several I/O requests and combines them into an aggregate “snapshot” of all write activity performed in the multiple I/O requests, and sends the snapshot to DPA 124, for journaling and for incorporation in target storage system 120. In snapshot mode DPA 112 also sends an acknowledgement to protection agent 144 upon receipt of each I/O request, before receiving an acknowledgement back from DPA 124.

For the sake of clarity, the ensuing discussion assumes that information is transmitted at write-by-write granularity.

While in production mode, DPA 124 receives replicated data of LU A from DPA 112, and performs journaling and writing to storage system 120. When applying write operations to storage system 120, DPA 124 acts as an initiator, and sends SCSI commands to LU B.

During a recovery mode, DPA 124 undoes the write transactions in the journal, so as to restore storage system 120 to the state it was at, at an earlier time.

As described hereinabove, LU B is used as a backup of LU A. As such, during normal production mode, while data written to LU A by host computer 104 is replicated from LU A to LU B, host computer 116 should not be sending I/O requests to LU B. To prevent such I/O requests from being sent, protection agent 164 acts as a target site protection agent for host Device B and fails I/O requests sent from host computer 116 to LU B through host Device B.

Target storage system 120 exposes a logical unit 176, referred to as a “journal LU”, for maintaining a history of write transactions made to LU B, referred to as a “journal”. Alternatively, journal LU 176 may be striped over several logical units, or may reside within all of or a portion of another logical unit. DPA 124 includes a journal processor 180 for managing the journal.

Journal processor 180 functions generally to manage the journal entries of LU B. Specifically, journal processor 180 enters write transactions received by DPA 124 from DPA 112 into the journal, by writing them into the journal LU, reads the undo information for the transaction from LU B. updates the journal entries in the journal LU with undo information, applies the journal transactions to LU B, and removes already-applied transactions from the journal.

Referring to FIG. 2, which is an illustration of a write transaction 200 for a journal. The journal may be used to provide an adaptor for access to storage 120 at the state it was in at any specified point in time. Since the journal contains the “undo” information necessary to roll back storage system 120, data that was stored in specific memory locations at the specified point in time may be obtained by undoing write transactions that occurred subsequent to such point in time.

Write transaction 200 generally includes the following fields: one or more identifiers; a time stamp, which is the date & time at which the transaction was received by source side DPA 112; a write size, which is the size of the data block; a location in journal LU 176 where the data is entered; a location in LU B where the data is to be written; and the data itself.

Write transaction 200 is transmitted from source side DPA 112 to target side DPA 124. As shown in FIG. 2, DPA 124 records the write transaction 200 in the journal that includes four streams. A first stream, referred to as a DO stream, includes new data for writing in LU B. A second stream, referred to as an DO METADATA stream, includes metadata for the write transaction, such as an identifier, a date & time, a write size, a beginning address in LU B for writing the new data in, and a pointer to the offset in the DO stream where the corresponding data is located. Similarly, a third stream, referred to as an UNDO stream, includes old data that was overwritten in LU B; and a fourth stream, referred to as an UNDO METADATA, include an identifier, a date & time, a write size, a beginning address in LU B where data was to be overwritten, and a pointer to the offset in the UNDO stream where the corresponding old data is located.

In practice each of the four streams holds a plurality of write transaction data. As write transactions are received dynamically by target DPA 124, they are recorded at the end of the DO stream and the end of the DO METADATA stream, prior to committing the transaction. During transaction application, when the various write transactions are applied to LU B, prior to writing the new DO data into addresses within the storage system, the older data currently located in such addresses is recorded into the UNDO stream. In some examples, the metadata stream (e.g., UNDO METADATA stream or the DO METADATA stream) and the data stream (e.g., UNDO stream or DO stream) may be kept in a single stream each (i.e., one UNDO data and UNDO METADATA stream and one DO data and DO METADATA stream) by interleaving the metadata into the data stream.

Referring to FIG. 3, an example of a replication system that includes asymmetric active-active access of asynchronously-protected storage is a system 300. However, it is important to note that any asynchronous replication may be suitable for the process of allowing asymmetric global cache and any continuous asynchronous replication may be suitable for failover. In the examples use herein DPA appliances are used, however the process may be performed by having appliance functionality performing at a virtual storage cluster.

The system 300 includes a primary storage volume 302 a (production volume) at a first site (production site) and a secondary storage volume 302 b (replica volume) at a second site (replication site). The system 300 also includes a DPA cluster 306 a at the first site, a DPA cluster 306 b at the second site, a first virtual storage cluster 308 a, a second virtual storage cluster 308 b and a global cache 310. In one example, the primary storage volume 302 a may be replaced with a consistency group across multiple storage volumes. In one example, the secondary storage volume 302 b may be replaced with multiple storage volumes.

The first virtual storage cluster 308 a includes a protection agent 320 a and a timer 330 a. The second virtual storage cluster 308 b includes a protection agent 320 b and a timer 330 b. The first and second virtual storage cluster 308 a, 308 b are referred to herein as global cache processing nodes.

The primary storage volume 302 a is asynchronously replicated to the secondary storage volume 302 b using the connection 318 and using the DPA clusters 306 a, 306 b. Each of the primary and secondary storage volumes is virtually realized using virtual storage volumes (e.g., the first virtual storage cluster 308 a and the second virtual storage cluster 308 b respectively). The global cache 310 provides asymmetric active-active access to the primary storage volume 302 a. Hosts (not shown) at the first and the second site are given access to the primary storage volume 302 a through the global cache 310. DPA cluster 306 a and DPA cluster 306 b are part of a replication layer.

The global cache 310 is a cache layer which manages a coherent cache between the two sites (e.g., the first and the second sites) allowing reads to be local at a remote site (e.g., devices at the second site can read the second storage volume) when possible.

The global cache exposes one virtual volume at both sites with the same identity i.e. on both sites the primary and secondary storage volumes 302 a, 302 b appear as the same SCSI entities to a host accessing the storage at both sites since the volumes 302 a, 302 b are not fully synchronized (i.e., primary storage volumes 302 a is kept more updated, and not all reads can be redirected at secondary storage volume 302 a).

The first virtual storage cluster 308 a performs synchronous reads and writes to the primary storage volume 302 a using the connection 312. The write operations to the primary storage volume 302 a is a local operation for the first virtual storage cluster 308 a and a higher latency remote operation from the second virtual storage cluster 308 b, which uses a proxy for the write through the first virtual storage cluster 308 a. It is this imbalance that makes the active-active access asymmetric.

The second virtual storage cluster 308 b can synchronously write to the first virtual storage cluster 308 a using the connection 314. While the reads from the primary storage volume are synchronous, the second virtual storage cluster 308 b can provide local performance (at the second site) by reading from the secondary storage volume 302 b using the connection 316. This local-read optimization is available only for regions on the secondary storage volume 302 b for which there has not been a recent write. The secondary storage volume 302 b typically lags behind the primary storage volume 302 a because the replication stream is asynchronous. The data that the secondary storage volume 302 b lacks to mirror the primary storage volume 302 a is called herein lagging data. As will be further described herein, to correctly determine which blocks are safe to read from the secondary storage volume, the global cache 310 is used to coordinate with a replication stream.

The DPA cluster 306 b at the second site writes replicated data to the secondary storage volume 302 b through a volume exposed by the second virtual storage cluster 308 b, but I/Os arriving though the DPA cluster 306 b flow directly to the secondary storage volume 302 b, (and do not go back to the first site through connection 314). The secondary virtual storage cluster 308 b identifies I/Os arriving from DPA as special I/Os either by identifying the initiator as a special DPA initiator, or by exposing a volume with a different SCSI identity to the DPA and having the DPA write through the different volume.

Write I/Os may be generated by host(s) at the second site. The I/O flows to the first site is then intercepted by the protection agent 320 a and split with the I/O data being stored at the primary storage volume 302 a replicated back to the second site to the secondary storage volume 302 b. In one example, a possible optimization can be to avoid sending the replicated I/O data from the first site back to the second site by keeping the data of the I/O at the second site in a memory and just sending metadata from the first site to the second site. The I/O data and the metadata are added to the second storage volume if the metadata is received form the first site.

FIGS. 4A and 4B are an example of a process to account for a lag in updating a secondary storage volume in a replication system having an asymmetric active-active access of an asynchronously-protected data storage by tagging writes with a phase value. An example of a process to tag writes with a phase value is a process 400.

Process 400 tags writes as belonging to a first phase (402). For example, writes from the global cache processing nodes are tagged with a first phase value indicating a first phase. In one particular example, the first phase is a phase n. In one example, writes may be tagged either by (1) adding a tag in some bits of the CDB (SCSI Command Descriptor Block, i.e. the metadata of the SCSI command) of the writes when the writes are sent to a DPA or (2) tags may be sent in parallel to the I/Os either through an SCSI or any other protocol, but in a manner that tags can be synchronized with the I/Os. One example is sending a new tag when I/Os are quiesced (i.e., I/Os are held in the storage processor and not acknowledged to the host while phase change occurs). Other protocols may require no quiescing.

Process 400 records, for each location written to on the primary storage volume and the secondary volume during the phase, the phase value (406). For example, the global cache 310 records, for each written location on the primary storage volume 302 a and the secondary storage volume 302 b, the phase value indicating the first phase. In one example, the written location is recorded using metadata.

After a period of time, process 400 notifies the global cache processing nodes of a phase change to a next phase (408). For example, the global cache 310 notifies the protection agent 320 a of the phase change and protection agent 320 a starts sending commands with the new phase in the metadata. In one particular example, the global cache 310 initiates a phase change periodically. In one example, the next phase is n+1.

Process 400 tags writes from the global cache processing nodes as belonging to the next phase (410). For example, the writes arriving from the protection agent 320 a to the DPA cluster 306 a and then sent to the DPA cluster 306 b are tagged as belonging to a n+1 phase. Even though a phase change is initiated the previous phase change is not complete until all writes belonging to the previous phase have completed, which means that the writes from two phases may be active concurrently (i.e., writes for each phase have not completed). For example, writes from a phase n and a phase n+1 may be active concurrently.

Process 400 determines if writes from a current phase are completed (416). If writes from a current phase are completed, process 400 proceeds to repeat processing block 408 for the next phase. For example, the next phase is n+2 phase. Note that now that phase n is closed the only active phase is phase n+1. After a period of time process 400 notifies the global cache processing nodes of a phase change to the next phase (408). For example, the protection agent 320 a is notified that the next phase is n+2.

An example of a process to process writes with tags is a process 450. Process 450 notifies the global cache after a write arrives at the second site and is distributed to the secondary storage volume (452). The replication layer (e.g., DPA clusters 306 a, 306 b) transfers writes in order to the second site. As the first write from a phase arrives at the second site and is distributed to the secondary storage volume 302 b, the replication layer (e.g., DPA clusters 306 a, 306 b), notifies the global cache 310.

Process 450 clear the phase value that has been recorded for each written location for phases that are two earlier than the phase being notified about (456). In one example, the global cache 310 clears the phase value that had been recorded for each written location in processing block 406 for phases that are two earlier than the phase being notified about. Thus if the notification is received that phase n+2 has started being distributed to secondary storage, the metadata can be cleared for written locations that were last written in phase n.

When the DPA cluster 306 b starts applying I/O requests from the newest phase to the secondary storage volume 302 b, the DPA cluster 306 b notifies the global cache 310. The notification is done either by a control path command or by adding the phase in the CDB of the write command.

When the Cache layer at the replica site accepts I/Os with phase n+2 it can safely assume that all I/Os of phases n and before are already flushed to the second site (replication site). And thus reads to locations for which the phase is n or lower can be served locally from the replica site.

When a read arrives at the second site a cache coherency layer (e.g. the global cache) checks whether it has any recorded phase value as might have been stored in 406, as this indicates that the data on the secondary storage volume is potentially older than the data on the primary storage volume (and the difference is still actively part of the replication stream). If so the read must be retrieved remotely from the primary site. If instead the global cache has no recorded phase value for the read location, that guarantees that the secondary storage volume contains the same data for this location as the primary storage volume, and the read data can come from the local secondary storage volume.

Referring to FIG. 4C, an example of a process to determine if a replicated storage volume can be read is a process 470. Process 470 receives a request to read a location on the secondary storage volume 302 b (472) and determines if the global cache 310 has a phase value for the read location (476). Process 470 reads the location at the second storage volume 302 b if the global cache does not have a phase value for the location (480). Process 470 reads the location at the first storage volume 302 a if the global cache 310 has a phase value for the location (486).

For example, when the global cache 310 is notified that phase n+2 has started to be distributed to the secondary storage volume 302 b, the global cache 310 recognizes that all writes from phase n and earlier must have completed distribution; however, there may still be writes from phase n+1 to come. Thus, the global cache 310 can mark the locations that have not been changed since phase n as mirrored locations, and allow future reads to those mirrored locations from the second virtual storage cluster 308 b to be read from the secondary storage volume 302 b. The global cache 310 ensures that writes to the primary storage volume 302 a that follow reads configured to use the secondary storage volume 302 b will wait for the earlier reads to be completed before being submitted to the storage or replication layers. Any write will access the metadata to determine whether the global cache 310 contains any data that will become stale because of this write. The write is handled at the storage cluster and when the storage node gets the write it accesses the metadata in the cache. This will include the data being read into cache from the secondary storage volume. The write invalidates the stale data to remove it from the cache, and will send an invalidation request to the reading node. The reading node will queue the invalidation request until the read completes.

Referring to FIG. 5, because the behavior of the system is asymmetric, in some cases there may be a desire to perform a failover, and transfer the primary site to the replica site. An example of a process to transfer primary storage functionality from one site to another site is a process 600. For example, process 500 is used to transfer the primary storage functionality from the first site to the second site (FIG. 3).

Process 500 decreases the lag between the primary and secondary storage volumes (502). For example, a replication layer (e.g., DPA clusters 306 a, 306 b) increases the replication rate, or decrease the incoming throughput of I/O requests to the first site by delaying acknowledgments to the writing hosts (not shown).

Process 500 determines if the lag of the secondary storage volume 302 b at the secondary site is below a predetermined threshold (506). For example, the replication layer (e.g., DPA clusters 306 a, 306 b) determines if the lag of the secondary storage volume 302 b is below a predetermined threshold. For example, the lag is smaller than 100 MB.

Process 500 notifies the global cache that an attempt can be made to transfer of primary storage functionality from the first site to the second site may be made (510). For example, the replication layer (e.g., DPA clusters 306 a, 306 b) notifies the global cache 310 of the transfer.

Process 500 suspends I/Os to the primary storage volume (516) and starts a timer (522). For example, the global cache 310 suspends the I/Os (i.e., write I/Os and read I/Os) to the primary storage volume 302 a and starts the timer 330 a in the storage cluster 308 a. In one example, the timer 330 a is a countdown timer of a length short enough to avoid an impact with an application using the primary storage volume 302 a.

Process 500 transfers the lagging data to the secondary storage volume (528). For example, the replication layer transfers the lagging data to the secondary storage volume 302 b in attempt to have the secondary storage volume 302 b mirror the primary storage volume 302 a prior to the timer expiring.

Process 500 determines if the timer expires before the primary storage volume and the second storage volume are mirrored (532). If the timer expires before the primary storage volume and the second storage volume are mirrored, process 500 allow I/Os to proceed to the primary storage volume (538) and notifies the replication layer that the transfer was aborted (542). For example, the global cache 310 allows the I/Os to proceed to the primary storage volume 302 a and notifies the DPA clusters 306 a, 306 b that the transfer from the first site to the second site was aborted.

If the timer does not expire before the primary storage volume and the second storage volume are mirrored, process 500 starts I/Os at the secondary storage volume (548) and notifies the replication layer that the transfer was a success (542). For example, the global cache 310 allows the I/O requests to start at the secondary storage volume 302 b and notifies the DPA clusters 306 a, 306 b that the transfer from the first site to the second site was a success.

The replication direction may now be reversed and replication is started from the second site and copied to the first site.

Referring to FIG. 6, an example of a process to automatically (without user intervention) transfer primary storage functionality from one site to another site is a process 600. For example, process 500 is used to automatically transfer the primary storage functionality from the first site to the second site (FIG. 3).

Process 600 detects writes from the first site and from the second site (602).

Process 600 determines if the second site with the secondary storage volume is providing more than a predetermined amount of writes than the first site with the primary storage volume 302 a (606).

If the second site is providing more than a predetermined amount of writes than the first site, process 600 performs the process 500 (FIG. 5) and transfers the primary storage functionality to the second site with the second storage volume 302 b (620). Thus, the secondary storage volume 302 b becomes the primary storage volume and the first storage volume 302 a becomes the secondary storage volume.

After process 500 is completed, process 600 returns to processing block 602.

It would be understood by one of ordinary skill in the art that instead of measuring the amount of writes the site with secondary storage volume provides more than the site with primary storage volume provides, a ratio between the writes provided at the site with secondary storage volume to writes provided at the site with primary storage volume may be measured and a ratio that exceeds a predetermined value would trigger execution of the process 500.

In order to perform a read from the secondary site to the replication volume, those areas of the replication storage volume must be synchronized with the primary storage volume. In particular, it is those areas within the replication storage volume being read from the second site that must be synchronized. Otherwise a read will be performed at the primary storage volume. Optimization of volume synchronization is important during first time initialization. When an asymmetric volume is generated and the replication site should be fully synchronized with the production site. Once the production volume and the replication volume are synchronized, data may be transferred to the replication site in a continuous way in the order of the arrival of the write, but when the production volume and the replica are not synchronized, synchronization order can be optimized.

Referring to FIG. 7, the primary storage volume 302 a and the replication storage volume 302 b can be divided into an equal amount of extents. An extent is a range of memory. In one example, an extent is 16 MB. In the example, depicted in FIG. 7, there are 25 extents. Thus to synchronize the replication volume 302 b with the primary storage volume 302 b each extent of the 25 extents must be sent from the primary storage volume 302 a to the replication storage volume 302 b.

Generally the synchronization process occurs one extent at a time. As depicted in FIG. 7 extents e₁, e₁₄ and e₁₅ in the replication storage volume 302 b are synchronized while e₂ is being synchronized. The remaining portions of the replication storage volume 302 b are blank and therefore not synchronized.

There are three types of extents: a read active extent, a not active extent and a write active extent. A read active extent has multiple reads for that extent and no or almost no writes for that extent during a period of time (e.g., a 1 hour window). A not active extent has neither reads nor writes for that extent. A write active extent has multiple writes for that extent. In one example, the read active extents are synchronized first, the not active extents are synchronized second and the write active extents are synchronized third.

Referring to FIG. 8, an example of a process to handle extents during synchronization is a process 800. Process 800 selects a next extent to synchronize (802). Process 800 sends the extent to the replication storage volume (806). Process 800 marks the extent as synchronized (808) and repeats processing block 808. In one example, DPA 306 a reads the extent statistics from the first virtual storage cluster 308 a to decide which extents should be read first. DPA 306 a reads the extents and writes them to the replica storage (secondary storage volume 302 b). When the external data is written to the replica storage (secondary storage volume 302 b) by the DPA 306 b, the DPA 306 b notifies the second virtual storage cluster 308 b that the extent is synchronized and from that time forward, the extent is treated as part of the standard synchronization mechanism, i.e. if the extent was written and has no phase value, the data corresponding to that extent can be read locally (i.e., a device at the second site can read the data from the second storage volume 302 b).

Referring to FIG. 9, an example of a process to select an extent for synchronization is a process 900. Process 900 determines an extent score (902) and selects the extent with the highest extent score for synchronization. In one example, the extent score is a difference of the number of reads to an extent versus the number of writes to the extent. In other examples, the number of writes and the number of reads to an extent are each weighted. In one particular example, the extent score is equal to: α·num_reads+β·num_writes, where α and β are constants, num_reads is the number of reads to an extent and num_writes is the number of writes to the extent. This extent score will be high if there are many reads and no writes and low if there are many writes. In one example, if α=1 and β=100, any writes will lower the score more than any writes. In one example, the global cache 310 determines the activity of each extent. In one example, the DPA 306 a reads the extent scores from the first virtual storage cluster 308 a.

Referring to FIG. 10, an example of a process to read from the second (replication) site is a process 1000. Process 1000 ensures a host at the second site may only read from the secondary storage volume locally if the portion of the secondary volume to be read is synchronized with the primary storage volume.

Process 1000 determines if an extent was synchronized (1002). For example, process 1000 determines if the extent was marked as synchronized in processing block 808.

If the extent is not synchronized, process 1000 performs the read at the primary storage volume (1010). If the extent is synchronized process 1000 determines if the extent is up to date (1012). For example, process 1000 determines if there is not a phase value to the extent and if so the extent is up to date. In one example, processing block is performed similar to process

If the extent is up to date, process 1000 reads from the secondary storage volume (1016). If the extent is not up to date, process 1000 reads from the primary storage volume (1014). Referring to FIG. 11, in one example, a computer 1100 includes a processor 1102, a volatile memory 1104, a non-volatile memory 1106 (e.g., hard disk) and the user interface (UI) 1108 (e.g., a graphical user interface, a mouse, a keyboard, a display, touch screen and so forth). The non-volatile memory 1106 stores computer instructions 1112, an operating system 1116 and data 1118. In one example, the computer instructions 1112 are executed by the processor 1102 out of volatile memory 1104 to perform all or part of the processes described herein (e.g., processes 400, 450, 470, 500, 600, 800, 900 and 1000).

The processes described herein (e.g., processes 400, 450, 470, 500, 600, 800, 900 and 1000) are not limited to use with the hardware and software of FIG. 11; they may find applicability in any computing or processing environment and with any type of machine or set of machines that is capable of running a computer program. The processes described herein may be implemented in hardware, software, or a combination of the two. The processes described herein may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a non-transitory machine-readable medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform any of the processes described herein and to generate output information.

The system may be implemented, at least in part, via a computer program product, (e.g., in a non-transitory machine-readable storage medium such as, for example, a non-transitory computer-readable medium), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers)). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a non-transitory machine-readable medium that is readable by a general or special purpose programmable computer for configuring and operating the computer when the non-transitory machine-readable medium is read by the computer to perform the processes described herein. For example, the processes described herein may also be implemented as a non-transitory machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate in accordance with the processes. A non-transitory machine-readable medium may include but is not limited to a hard drive, compact disc, flash memory, non-volatile memory, volatile memory, magnetic diskette and so forth but does not include a transitory signal per se.

The processes described herein are not limited to the specific examples described. For example, the processes 400, 450, 470, 500, 600, 800, 900 and 1000 are not limited to the specific processing order of FIGS. 4A to 4C, 5, 6 and 8 to 10, respectively. Rather, any of the processing blocks of FIGS. 4A to 4C, 5, 6 and 8 to 10 may be re-ordered, combined or removed, performed in parallel or in serial, as necessary, to achieve the results set forth above.

The processing blocks (for example, in the processes 400, 450, 470, 500, 600, 800, 900 and 1000) associated with implementing the system may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field-programmable gate array) and/or an ASIC (application-specific integrated circuit)). All or part of the system may be implemented using electronic hardware circuitry that include electronic devices such as, for example, at least one of a processor, a memory, a programmable logic device or a logic gate.

Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Other embodiments not specifically described herein are also within the scope of the following claims. 

What is claimed is:
 1. A method comprising: synchronizing a second storage volume to a first storage volume by synchronizing extents in order according to priority based on extent activity; and selecting an extent to synchronize based on an extent score, the extent score measures the difference between reads to the extent and writes to the extent, wherein synchronizing the second storage volume to the first storage volume by synchronizing extents in order according to priority comprises synchronizing read active extents and not active extents prior to synchronizing write active extents, wherein selecting an extent to synchronize based on an extent score comprises selecting an extent to synchronize based on an extent score equal to: α·num_reads−β·num_writes, where α and β are constants, num_reads is the number of reads to the extent and num_writes is the number of writes to the extent.
 2. The method of claim 1, further comprising determining an extent in the second storage volume may be read if the extent is synchronized and is up to date.
 3. The method of claim 2, further comprising determining the extent in the second volume cannot be read and the extent in the first volume should be read instead if one of the extent is not synchronized or the extent is not up to date.
 4. The method of claim 1, further comprising selecting an extent with the highest extent score to synchronize first.
 5. The method of claim 1, wherein selecting the extent to synchronize based on the extent score comprises selecting an extent to synchronize based on an extent score weighting the number of reads to the extent and weighting the number of writes to the extent.
 6. An apparatus, comprising: electronic hardware circuitry configured to: synchronize a second storage volume to a first storage volume by synchronizing extents in order according to priority based on extent activity; and select an extent to synchronize based on an extent score, the extent score measures the difference between reads to the extent and writes to the extent, wherein the circuitry configured to synchronize the second storage volume to the first storage volume by synchronizing extents in order according to priority comprises circuitry configured to synchronize read active extents and not active extents prior to synchronizing write active extents, wherein the circuitry configured to select an extent to synchronize based on an extent score comprises circuitry configured to select an extent to synchronize based on an extent score equal to: α·num_reads−β·num_writes, where α and β are constants, num_reads is the number of reads to the extent and num_writes is the number of writes to the extent.
 7. The apparatus of claim 6 wherein the circuitry comprises at least one of a processor, a memory, a programmable logic device or a logic gate.
 8. The apparatus of claim 6, further comprising circuitry configured to determine an extent in the second storage volume may be read if the extent is synchronized and is up to date.
 9. The apparatus of claim 8, further comprising circuitry configured to determine the extent in the second volume cannot be read and the extent in the first volume should be read instead if one of the extent is not synchronized or the extent is not up to date.
 10. The apparatus of claim 6, wherein the circuitry configured to select the extent to synchronize based on the extent score comprises circuitry configured to select an extent to synchronize based on an extent score weighting the number of reads to the extent and weighting the number of writes to the extent.
 11. An article comprising: a non-transitory computer-readable medium that stores computer-executable instructions, the instructions causing a machine to: synchronize a second storage volume to a first storage volume by synchronizing extents in order according to priority based on extent activity; and select an extent to synchronize based on an extent score, the extent score measures the difference between reads to the extent and writes to the extent, wherein the instructions causing the machine to synchronize the second storage volume to the first storage volume by synchronizing extents in order according to priority comprises instructions causing the machine to synchronize read active extents and not active extents prior to synchronizing write active extents, wherein the instructions causing the machine to select an extent to synchronize based on an extent score comprises instructions causing the machine to select an extent to synchronize based on an extent score equal to: α·num_reads−β·num_writes, where α and β are constants, num_reads is the number of reads to the extent and num_writes is the number of writes to the extent.
 12. The apparatus of claim 6, further comprising circuitry configured to select an extent with the highest extent score to synchronize first.
 13. The article of claim 11, wherein the instructions causing the machine to select the extent to synchronize based on the extent score comprises instructions causing the machine to select an extent to synchronize based on an extent score weighting the number of reads to the extent and weighting the number of writes to the extent.
 14. The article of claim 11, further comprising instructions causing the machine to determine an extent in the second storage volume may be read if the extent is synchronized and is up to date.
 15. The article of claim 14, further comprising instructions causing the machine to determine the extent in the second volume cannot be read and the extent in the first volume should be read instead if one of the extent is not synchronized or the extent is not up to date.
 16. The article of claim 11, further comprising instructions causing the machine to select an extent with the highest extent score to synchronize first. 